Solvent-based adhesives have been in use for joining thermoplastic pipe for over 30 years. The development of these adhesives is largely responsible for growth of the thermoplastic pipe industry. Several billion pounds of plastic pipe are produced each year in North America. Rapid-setting, solvent-based adhesives are used to bond the pipes together in a timely manner, as well as to bond thermoplastic pipe fittings (e.g., tees, ells, caps, couplings, and so forth) to such pipe. These rapid-setting, solvent-based adhesives allow for the testing and trouble-shooting of piping systems in a matter of hours while maintaining the long-term durability of pipe itself. These characteristics, rapid-set, ease of use, long-term durability along with low-cost, have made the joining of plastic pipe by solvent-based adhesives a practical and economic system.
The solvent-based adhesives generally are formulated by dissolution of a relatively small amount of resin solids in a volatizable solvent carrier. The solvent-based adhesives work primarily by two means of action. First, upon surface application to a thermoplastic pipe surface, the solvent portion of the solvent-based adhesive formulation softens (i.e., partially dissolves) the outer surfaces of the pipe through solvation of the plastic such that the plastic surface becomes tackified. Subsequently, the adhesive "cures", i.e., hardens, by evaporation of the solvent fraction into the surrounding atmosphere from the pipe. Before the adhesive cures completely, the adhesive-treated plastic surface is brought into contact with the surface of another plastic piece to which it will be surface-attached, often under pressure. Secondly, the resin dissolved in the adhesive formulation remains at the plastic joint location and dries through the solvent evaporation and diffusion to ultimately provide a continuous joint between the bonded pipe surfaces, which aids in preserving the integrity of the entire pipe system. The term "adhesive", as as used in this specification and in the claims herein, whether in its singular or plural forms, includes cements, glues and pastes.
These solvent-based adhesives cure rapidly (viz., within a matter of hours), often permitting piping systems to be tested the same day as constructed. However, perhaps the most important benefit of these solvent-based adhesives is the maintenance of the integrity of the pipe itself. The resin solids component of the solvent-based adhesive can be selected so as to match the plastic pipe material and or plastic fitting material. This provides for a high degree of long-term durability for the piping system, often up to 30 to 40 years of useful life. This long service-life is essential for piping systems which are built into long-term structures such as homes and buildings, or where buried underground.
Thousands of miles of thermoplastic piping systems are constructed each day throughout the world, primarily by the means previously described. These systems are used in the transfer of potable water for drinking; residential hot and cold water systems; drain, waste, and vent (DWV) applications in home and industry; turf and agricultural sprinkler systems; Jacuzzi, spa and tub connections, residential and commercial fire sprinkler systems; and so forth. These systems are crucial to the maintenance of safe and healthy means of transport of water and other chemicals throughout the world.
Evaporation of organic solvents from adhesives, as well as paints, coatings and the like, has created concerns from an environmental perspective, as it represents a source of air pollution. Typical solids (non-volatile) contents of thermoplastic solvent-based pipe adhesives are 10 to 20% for CPVC, with the balance of the formulation constituted primarily by solvent. Thus, current adhesives and primers incorporate the use of highly volatile solvents as major portions of the formulations. Among these, tetrahydrofuran (THF) is highly volatile at 20.degree. C. with a vapor pressure of 143 mm Hg. Co-solvents currently used are acetone (vapor pressure of 185 mm Hg), and methyl ethyl ketone (vapor pressure of 70 mm Hg). While other solvents, such as cyclohexanone (vapor pressure of 2 mm Hg at 20.degree. C.), may also be used in such solvent-based plastic pipe adhesives, the amounts of such solvents are comparatively minor, namely, considerably less than 10 wt. %, of the total adhesive formulation.
Many of these solvents typically used in thermoplastic solvent-based pipe adhesives, e.g., tetrahydrofuran (THF), methyl ethyl ketone (MEK), and acetone, are considered to be volatile organic compounds ("VOC", calculated in accordance with ASTM D-2369). That is, VOCs are defined as volatile compounds which contain the element carbon, excluding compounds such as methane, carbon monoxide, carbon dioxide, carbonic acid, metallic carbides and carbonates, ammonium carbonates, as well as certain exempted solvents such as methylene chloride and 1,1,1-trichloroethane. Typical VOC values of present commercial CPVC solvent-based adhesives range from 775 to 850 grams/liter (g/L), and the VOC composite partial pressures are between 120 and 160 mm Hg at 20.degree. C.
THF is a heterocyclic compound that is readily commercially available. THF is a very efficient solvent in CPVC and PVC adhesives because of its effective dissolution of CPVC and PVC resins and its relatively high vapor pressure that allows fast bond formation. As a consequence, adhesive formulations employing THF as a solvent yield pipe joints that are very strong even under hydrostatic pressure. Another heterocyclic compound, N-methyl pyrolidine (NMP), also has been used in conventional PVC adhesives as it effectively dissolves PVC and CPVC resins. However, NMP has a relatively low vapor pressure and, consequently, it normally must used in combination with a higher vapor pressure solvent or solvents, such as THF, in order to develop effective bond strength in a reasonable period of time. However, specific environmental concerns recently have been raised regarding the high content levels of THF present in many current solvent-based adhesive formulations, such as plastic pipe formulations.
For these reasons, it would be highly desirable to develop CPVC adhesive formulations that completely omit heterocyclic solvents, or at least significantly limit the content of heterocyclic solvents, such as THF, while still meeting performance standards applicable to thermoplastic pipe. Namely, such adhesives must meet the required hydrostatic burst strength and other performance standards applicable to CPVC pipe adhesives, such as set forth in ASTM F-493 and D-2846.